154 THE DESCENT OF MAN. Part I. 



naturally follow from the assumed uniformity of the 

 exciting causes, and likewise from the free intercros- 

 sing of many individuals. The same organism might 

 acquire in this manner during successive periods suc- 

 cessive modifications, and these would be transmitted 

 in a nearly uniform state as long as the exciting causes 

 remained the same and there was free intercrossing. 

 With respect to the exciting causes we can only say, as 

 when speaking of so-called spontaneous variations, that 

 they relate much more closely to the constitution of the 

 varying organism, than to the nature of the conditions 

 to which it has been subjected. 



Conclusion. — In this chapter we have seen that as man 

 at the present day is liable, like every other animal, to 

 multiform individual differences or slight variations, so 

 no doubt were the early progenitors of man ; the varia- 

 tions being then as now induced by the same general 

 causes, and governed by the same general and complex 

 laws. As all animals tend to multiply beyond their 

 means of subsistence, so it must have been with the 

 progenitors of man ; and this will inevitably have led 

 to a struggle for existence and to natural selection. 

 This latter process will have been greatly aided by 

 the inherited effects of the increased use of parts; 

 these two processes incessantly reacting on each other. 

 It appears, also, as we shall hereafter see, that various 

 unimportant characters have been acquired by man 

 through sexual selection. An unexplained residuum 

 of change, perhaps a large one, must be left to the 

 assumed uniform action of those unknown agencies, 

 which occasionally induce strongly-marked and abrupt 

 deviations of structure in our domestic productions. 



Judging from the habits of savages and of the greater 

 number of the Quadrumana, primeval men, and even 



